1. Field of the Invention
This invention relates to a hard disk drive. Specifically, this invention relates to an adaptive feed forward device for reducing current control errors.
2. Description of the Related Art
An earlier servo controller having a feed forward device which reduces control errors during the servo control of a hard disk drive is composed of a plant which is an object of the servo control, an angle to position converter, a digital signal processor for servo controlling the plant, a digital to analog converter, and an adder. The plant generates an angle value which is sent to the angle to position converter which converts the angle information into position information which is then sent to the digital signal processor.
The digital signal processor is composed of an estimator, a velocity commander and an adder, a gain controller, and a feed forward controller for reducing control errors.
The estimator receives position information from the angle to position converter and a previous current command value from the feed forward controller through the gain controller and then calculates a position estimation value, a velocity estimation value, and a disturbance estimation value using predetermined estimation functions. The velocity commander sends a velocity command value to the adder in response to the position estimation value and the adder calculates a velocity error by subtracting the velocity estimation generated by the estimator from the velocity command value from the velocity commander. The velocity error is sent to the gain controller which receives the velocity from the adder and both the position estimation value and the disturbance estimation value from the estimator. The gain controller produces gains according to a predetermined formula represented by predetermined gain transfer functions. The outputs of the gain controller are current command values for servo control of the plant. The feed forward controller is connected to the output of the gain controller to reduce current control errors. The current command value is generated only after the current control errors have been reduced by feed forward controller.
The feed forward controller includes a memory, a switch, and an adder. The memory contains a predetermined fixed feed forward value. The switch switches on and off according to an external feed forward control and the adder sums the output of the gain controller and the predetermined feed forward value when switch is closed, resulting in the current command value. The current command value is converted to an analog value by the digital to analog converter which generates a driving current value for the plant. The plant is driven in response thereto.
The feed forward controller is necessary in that the current command value may have negative values which approach zero at the end of deceleration portion of a servo control. To generate a negative current command value without utilizing a feed forward controller, the gain controller must increase the gain of a signal by a great amount. However, as the gain increases, noise also increases.
To decrease noise caused by the current control, the feed forward controller is utilized. The feed forward value, which is constant in earlier servo controllers, represents a deceleration and is supplied to the adder of the feed forward controller during a predetermined portion of the deceleration. The switch of the feed forward controller is closed by the external feed forward control during this predetermined portion.
Since the feed forward value is supplied to the adder during the predetermined portion of the deceleration, the gain controller does not have to increase the signal gain and therefore, the noise contained in the current command value, generated by the digital signal processor, does not increase and the noise caused by the current control decreases.
Although the above noted use of the feed forward technique decreases the noise resulting from the current control, the use of a fixed feed forward value causes a current control error which causes problems of its own.
The patents to Yoneda et al., Iwashita, and Hanks, U.S. Pat. Nos. 5,313,147, 5,406,182 and 5,548,192, respectively entitled Digital servo-Control Apparatus, Method For Controlling A Servomotor, and Adaptive Feedback System For Controlling Head/Arm Position In A Disk Drive, each disclose control systems having adaptive feed forward devices. However, these patents fail to teach or suggest the adaptive feed forward technique of the present invention.
The following additional patents each disclose features in common with the present invention but do not teach or suggest the specific adaptive feed forward technique of the present invention.
U.S. Pat. No.5,592,345 to Carobolante et al., entitled Feedforward Circuit And Method For Controlling The Speed Of A Spindle Motor In A Disk Drive, U.S. Pat. No. 5,248,921 to Kato et al., entitled Servo Motor Control Device, U.S. Pat. No. 5,270,627 to Rehse, entitled Machine Tool Control system, U.S. Pat. No. 5,272,423 to Kim, entitled Velocity Control Method For A Synchronous AC ServoMotor, U.S. Pat. No. 5,305,158 to Ueda et al., entitled Positioning Control Apparatus, U.S. Pat. No. 5,073,747 to Dupraz et al., entitled Feedback System Control Device And Applications In Amplifiers And Servomechanisms, U.S. Pat. No. 5,473,230 to Dunn et al., entitled Torque Disturbance Precompensation For Disk Drive Spindle Motors, U.S. Pat. No. 5,301,072 to Wilson, entitled Sample Data Position Error Signal Detection For Digital Sector Servo, U.S. Pat. No. 5,204,594 to Carobolante, entitled Circuit For Providing A Signal Proportional To The Average Current Flowing Through Coils Of A Motor Operated In Both Linear And PWM Modes, U.S. Pat. No. 5,184,257 to Koga et al., entitled Head Positioning Control For A Spindle Motor Disk Drive, U.S. Pat. No. 5,126,689 to Nakamura, entitled Direct-Coupled Grounded-Base Amplifier, Semiconductor Device And Information Processing Device Having The Amplifier Therein, U.S. Pat. No. 4,789,975 to Taniyama, entitled Apparatus For Recording And/Or Reproducing Data Signal On Or From Disk Shaped Recording Medium At A Variably Selected Constant Linear Velocity, and U.S. Pat. No. 4,669,004 to Moon et al., entitled High Capacity Disk File With Embedded Sector Servo.